Recently, multimedia data such as music data and image data is desired to be exchanged between the devices by mutually connecting devices such as a mobile telephone, audio visual device and personal computer. To be more specific, it is anticipated to manage music data recorded by an audio device in a personal computer or transfer image data recorded by a visual device to a mobile telephone and view the image data outside.
UWB (Ultra Wide Band) for transmitting pulse signals in a wide frequency band is focused upon as such a data communication scheme between devices. Taking into account the characteristic of UWB of transmitting pulse signals, the OOK modulation scheme is optimal for transmitting data depending on whether or nor there is a pulse.
However, even if signals are modulated and transmitted according to the OOK scheme, multipath propagation due to reflection, scattering and diffraction occurs depending on the surrounding environment and delay waves arrive causing deterioration of signals. Such a situation will be described with reference to FIG. 1.
Transmission signal D100 shown in FIG. 1 is subjected to OOK modulation at the transmitting end and is encoded into symbols of “1” or “0” depending on whether or not there is a pulse.
On the other hand, although transmission signal D100 is received at the receiving end as received signal D101, delay waves arrive and interference occurs due to the main waves and delay waves. For example, in FIG. 1, although the two “1” symbol intervals (encircled portions) in received signal D101 are fundamentally “0” symbol intervals, these symbol intervals are encoded to “1” by error because two delay waves TF 3 and TF 4 have arrived.
The above conventional method for avoiding a code error due to the delay waves is disclosed (for example, Patent Document 1).
FIG. 2 shows a conventional example of transmission signal (PPM (Pulse Position Modulation) signal) D111 and received signals D112, D113 and D114. A case of a four-pulse position modulation is illustrated here. In this case, each code is divided into four different time slots S1 to S4 and each time slot shows digits of each code. Further, only one of the four different time slots has a voltage level representing binary 1.
Three received signals D112 to D114 shown in FIG. 2 are the direct wave and delay waves of transmission signal D111 received at the receiving end, and received data signal D115 is data multiplexed by superimposing these received signals D112 to D114. Consequently, received data signal D115 has a wider pulse width than transmission signal D111, thereby producing a bit error upon a demodulation.
However, by carrying out the processing shown in FIG. 3, for example, the above bit error is avoided. To be more specific, as shown in FIG. 3A, transmission signal D111 is re-encoded according to the relationship between codes and pulse positions. For example, in FIG. 3A, a combination of codes is shown in which “11” is followed by “00” (see the upper part), and this combination of codes is re-encoded to “0100” (see the lower part), so that the pulse width is reduced to half.
Further, as shown in FIG. 3B, a recovered signal obtained by recovering the received signal is re-encoded based on the combination of codes in the recovered signal. Furthermore, the received signal here refers to the code delay (i.e. extended portion) of received data signal D115 of FIG. 2 of transmission signal D111. For example, in FIG. 3B, if there is a combination of codes in which “0001” is followed by “0000” in the recovered signal, the recovered signal is re-encoded to “00011000.”
By carrying out such re-encoding processing, even if a pulse width becomes wider due to the influence of delay waves, it is possible to avoid a code error due to delay waves.
Patent Document 1: Japanese Patent Application Laid-Open No. 2004-229288 (paragraph [0033] to [0037], FIG. 4, FIG. 5 and FIG. 8)
Non-Patent Document 1: “Signpost to Shannon limit: A tutorial on “parallel concatenated (Turbo) coding,” “Turbo (iterative) decoding” and related topics,” Motohiko Isaka and Hideki Imai, Institute of Electronics, Information and Communication Engineers, IEICE technical report, IT98-51, December 1998.Non-Patent Document 2: “Low density parity check code and decoding method” TRICEPSNon-Patent Document 3: “Impress standard textbook series, improved edition, 802.11 high-speed wireless LAN textbook,” Impress CorporationNon-Patent Document 4: “Digital wireless transmission technique,” Pearson EducationNon-Patent Document 5: “Maximum Likelihood Decoding of Convolutional Codes and Its Performance Characteristics,” Yutaka Yasuda and Yasuo Hirata, Institute of Electronics, Information and Communication Engineers A, Vol. J73-A, No. 2, pp. 218 to 224